The present invention relates to a frequency control technology of a filter circuit, and to a technology effective if applied to a case in which the frequency of a filter to be controlled is controlled by a PLL loop including a dummy filter identical in configuration to the filter, e.g., a technology effective if applied to a wobble signal extracting circuit used in a DVD (Digital Versatile Disk) device.
In a disk type record playback device like a DVD-RAM (Random Access Memory), there have heretofore been known rotation control called a CLV (Constant Linear Velocity) system which makes constant a velocity, i.e., a linear velocity of a pickup relative to a disk to thereby perform read/write, and rotation control called a CAV (Constant Angular Velocity) system which makes constant a rotational velocity of a disk regardless of the position of a pickup to thereby perform read/write.
The device having adopted the CLV system has an advantage in that while the rotation control becomes complex because the rotational velocity of the disk must be changed according to a track position of the pickup in order to make the linear velocity of the pickup constant, a processing speed at a data processing circuit like a DSP (Data Processor) for processing a read signal may be constant, thus making it possible to simplify a signal processing system.
On the other hand, in the device having adopted the CAV system, the rotation control becomes simple because the rotational velocity of the disk may be controlled constant regardless of a track position of the pickup. However, since the relative velocity of the pickup, i.e., the frequency of a read signal changes depending on the position of the pickup in a diameter direction, the signal-processing speed at the data processing circuit must be changed and hence a signal processing system becomes complex.
As a recordable/reproducible optical disk, there has heretofore been known one formed with wobble defined as an index for swelling or waving an information recording track of a disk surface to thereby fetch out position information. In a DVD device which effects record/playback on a disk having wobble, a rough area of a track or a sector or the like is determined or judged based on address information pre-written in the disk, and a detailed recording position is detected by the wobble, whereby such control as to write data therein is performed.
In order to detect the wobble on the disk surface and thereby generate a clock signal in the DVD device of such a control system, a signal read from a pickup is passed through a bandpass filter to remove noise and thereafter digitized by a comparator or the like, thereby generating the corresponding clock signal. In order to accurately detect the wobble, the bandpass filter that extracts a wobble signal from the read signal is required to have a narrow-band filter characteristic.
Meanwhile, in an optical disk type record playback device, the rotational velocity of a disk might be changed stepwise like an equi- or same-speed or a quad-speed. Since the frequency of the wobble signal varies depending on a double-speed mode, there is a need to switchably set the center frequency of the bandpass filter according to the double-speed mode. As a control system for changing the center frequency of the bandpass filter, there has heretofore been proposed a system for controlling an intermediate frequency of a bandpass filter by a frequency follow-up control loop including a dummy filter identical in configuration to a filter to be controlled, and a phase comparator (see, for example, a patent document 1 shown below).
Japanese Unexamined Patent Publication No. 2000-231726